1. Field of the Invention
The present invention relates to a method for operating a magnetic resonance tomography device with a gradient system having at least one gradient coil arrangement for generating a gradient field in a spatial direction and which contains an energy supply device that is connected to the gradient coil arrangement; the gradient coil arrangement having at least one first sub-coil and one second sub-coil, and wherein the energy supply device is fashioned such that the currents in the sub-coils can be adjusted independently of one another.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for acquiring images of the inside of the body of a living examination subject. For this purpose, rapidly switched magnetic gradient fields, which have a high amplitude and which are generated by a gradient system, are superimposed on a static basic magnetic field.
The gradient system includes gradient coils, gradient amplifiers and a gradient control. One of the gradient coils, for a specific spatial direction, generates a gradient field having a gradient, which, at least within an imaging volume of the magnetic resonance tomography device, is approximately of the same magnitude in a location-independent manner at any arbitrary point in time. Since the gradient field is a chronologically variable magnetic field, the aforementioned is still valid for any point in time but the magnitude is variable from one point in time to another point in time. Normally, the direction of the gradient is strictly prescribed by the gradient coil design.
The currents are adjusted in the gradient coil for generating the gradient field. The amplitudes of the required currents amount to several 100 A. The current rising and falling rates (xe2x80x9cslew ratexe2x80x9d) amount to several 100 kA/s. The gradient coil is connected to a gradient amplifier for the current supply. Since the gradient coil represents an inductive load, high initial voltages of the gradient amplifier are necessary for generating the aforementioned currents.
In the case of magnetic resonance image pickups in living examination subjects, unwanted stimulations in the examination subject can be triggered due the switching of the gradient fields. The gradient fields thereby have an effect on the examination subject and are characterized by a chronologically changing magnetic flux density, which generates eddy and inductance currents in the examination subject.
Methods are known for predicting these stimulations. One of these methods for monitoring the stimulation is based on the dB/dt model, for example. In this method, the values of the chronological change of the magnetic flux density (dB/dt-values) of gradient fields are controlled and monitored, these values occurring during magnetic resonance tomography. The maximally allowable dB/dt values derive from the result of a stimulation study with the corresponding gradient coil, or from the limiting values that are strictly prescribed by the facility operating the tomography apparatus, for example. Further details are provided by J. Abart et al. xe2x80x9cPeripheral Nerve Stimulation by Time-Varying Magnetic fieldsxe2x80x9d, J. Computer Assisted Tomography (1997) 21 (4), pages 532 to 538.
The initiation of stimulations essentially depends on the type of pulse sequence employed in the imaging. Such sequences are broadly differentiated between conventional sequences and the fast sequences. Normally, conventional sequences require a high linearity of the gradient fields within a specific linearity volume, for example a linearity of approximately 5% in a spherical linearity volume having a diameter of approximately 40 to 50 cm given moderate gradient intensities of 10 to 20 mT/m and switching times of approximately 1 ms. High gradient intensities, e.g. 20 to 40 mT/m, are extremely rapidly switched for the fast sequences (switching times circa 100 to 500 xcexcs). The time-varying magnetic flux density of the gradient fields induces electric currents in the examination subject, and these electric currents can initiate stimulations of the examination subject. As a result of faster time variations, i.e., faster switching times and higher values of the magnetic flux density of gradient fields, the induced currents become larger and the likelihood of stimulations increases. Values that are the largest in terms of magnitude are reached at the edges and outside of the linearity volumes; this is where the maximal field boost occurs. Given the requirements to be met by the gradient intensity and the switching time, the boost is reduced and therefore the risk of stimulation because a gradient coil having a smaller linearity volume is utilized. Therefore, the linearity volume is reduced to a diameter of 20 cm, for example, in fast sequences. Normally, a gradient coil with the aforementioned properties for fast sequences is not suitable for conventional whole body applications, but it is suitable for magnetic resonance imaging techniques, such as the echo planar method and its hybrids.
Published German application OS195 40 746 describes a modular gradient coil system, which has two gradient coils for a spatial direction. One of the two coils or a series connection of both gradient coils is optionally connected to a gradient amplifier. For example, only one of the gradient coils is used for fast sequences and the series connection is used for conventional sequences. The gradient coil system thereby has a small linearity volume for fast sequences and allows the fast switching of gradient fields with large gradient intensities. Given the common operation of both coils, the gradient coil system has a larger linearity volume for conventional sequences with slowly switched gradient fields and with respect to smaller gradient intensities. A disadvantage of the aforementioned gradient coil system is that the size of the linearity volume and the quality of the linearity can be varied only in three steps at a maximum.
It is an object of the present invention to provide a method for operating a magnetic resonance tomography device of the type described above which improves the avoidance of producing stimulations of a living examination subject.
This object is achieved in accordance with the invention in a method for operating a magnetic resonance tomography device having a gradient system, which contains at least one gradient coil arrangement for generating a gradient field in a spatial direction and which contains an energy supply device that is connected to the gradient coil arrangement, wherein the gradient coil arrangement has at least one first sub-coil and one second sub-coil and wherein the energy supply device is fashioned such that currents can be adjusted independently of one another in the sub-coils, and wherein, for the continuous adjustment of at least one property of the gradient field, the current in at least one of the sub-coils is determined and adjusted by solving an optimization task containing a target function and at least one secondary condition, so that stimulations of a living examination subject are avoided.
For example, stimulations are prevented when an extreme value of the magnetic flux density of the gradient field remains below a fixable stimulation limiting value given a fixed slew rate of a sequence. The optimization task is solved by a variation calculation, for example. The target function contains coefficients of a spherical function development of a magnetic flux density of the gradient field, and the target function contains coefficients for each of the sub-coils. The coefficients for one of the sub-coils are multiplied with a factor that corresponds to a ratio of an adjustable current to a nominal current of the sub-coil. For example, a further secondary condition is that at least one of the coefficients multiplied with the factor is larger than a fixed limiting value. For example, it is possible to prescribe a minimally required gradient intensity with this version of the invention.
In addition to a current adjustment for operating the gradient system, a design of the gradient system also can be determined by solving the aforementioned optimization task. In the design of the gradient coil arrangement, coefficients and necessary nominal currents of the sub-coils are determined from properties of the gradient field prescribed in areas, in combination with other design criteria such as rigid body movement of the gradient coil arrangement in the device, eddy current behavior, noise generation etc. On the basis of a fixed gradient coil arrangement with fixed coefficients and nominal currents, the currents to be adjusted are determined dependent on the desired properties of the gradient field by means of the determination of current adjustments.
In an embodiment, at least one of the sequence parameters is prescribed for a selected sequence type in an area, so that the sequence can be executed by means of the gradient system of the device. The sequence parameter thereby co-determines at least one of the properties of the gradient field. For example, the minimally required gradient intensity and the minimally required slew rate is fixed in that a sequence type is selected. Furthermore, the selection of a field of views, for example, directly effects the size of the linearity volume. Since the user can only select parameters that can be executed by means of the gradient system of the device, the prescription of parameters that cannot be executed is avoided.
In another embodiment, the sequence parameter can be varied during the execution of the sequence.
In another embodiment, a current effecting a first linearity and/or a first linearity volume and/or a first gradient intensity of the gradient field is adjusted in the first sub-coil, and the current in the second sub-coil is controlled such that the first linearity and/or the first linearity volume and/or the first gradient intensity can be continuously varied. The current in the second sub-coil can be controlled for the purpose as to size and polarity sign.
In another embodiment, the currents are adjusted in the sub-coils such that stimulations of a living examination subject are prevented. Since the properties of the gradient field can be continuously adjusted at least in most areas, it is possible in an adjustment that avoids stimulations to always remain slightly below a stimulation threshold, so that the gradient system is operated with optimum efficiency.
In another embodiment, the energy supply device contains a first gradient amplifier, which is connected to the first sub-coil, and a second gradient amplifier, which is connected to the second sub-coil. As a result of the utilization of known and proven components of a magnetic resonance tomography device, the energy supply device is fashioned in a simple way such that the currents in the sub-coils can be adjusted independently of one another.
In another embodiment, the first sub-coil is fashioned for a specific linearity volume of the gradient field and/or for a specific linearity of the gradient field, and the second sub-coil is fashioned as a correction coil, so that the specific linearity volume and/or the specific linearity can be varied. As a result, the specific linearity volume can be enlarged and the specific linearity can be improved via the linearity volume for conventional sequences, in particular. The linearity volume, which is spherical for example, is the volume within which the linearity of the gradient field does not exceed a fixed linearity deviation, indicated in percent, for example. Therefore, the linearity of the gradient field characterizes the quality of the gradient field within the linearity volume, whereby the quality is described by the linearity deviation. A linearity deviation of 0% means that the curve of the gradient field is ideally linear within the linearity volume.
In another embodiment, the first sub-coil is fashioned for a specific gradient intensity of the gradient field, with the gradient intensity preferably being the maximum gradient intensity in terms of magnitude for the first sub-coil, and the second sub-coil is fashioned as an amplification coil, so that the specific gradient intensity can be varied, preferably increased. Therefore, the high gradient intensities that are particularly necessary for the fast sequences can be reached.